Conventionally, OFDM (Orthogonal Frequency Division Multiplexing) systems have been known as schemes having a tolerance to frequency selective fading, because communicated information is divided into a plurality of subcarriers, and the signals are orthogonal to one another. Further, a guard interval (hereinafter, “guard interval” is also referred to as “GI”) is provided to eliminate the effect of the preceding symbol due to the delayed versions. As the length of a guard interval is longer, the tolerance to delayed versions with long delay time is higher, while the rate enabling transmission of information is lower.
For example, for a data symbol of 50 μs, when the guard interval is 5 μs (microseconds), the effective rate is 50/55, and when the guard interval is 10 μs, the effective rate is 50/60, resulting in a reduction in throughput of communication. In other words, in systems such as the OFDM system and the like requiring the guard interval, when the guard interval is set at a length of a delayed version with the longest delay difference time, it is possible to perform communications without interference from delayed versions and waste time. FIGS. 11 and 12 show an example of the relationship between delayed versions and guard interval. FIG. 11 shows the case where multipath delay is smaller than GI, and FIG. 12 shows the case where multipath delay is larger than GI. In FIGS. 11 and 12, a frame configuration in transmission is shown on the top, and frame configurations in reception are shown below. The diagonally right up shaded areas shown in portions under the frames indicate ranges enabling division of signals without problems, and the diagonally right down shaded areas indicate portions where the signal deteriorates due to interference from the preceding data. When multipath delay is larger than GI, the symbol (signal) degrades due to interference from the preceding data.
Further, the relationship between the GI length, interference and transmission rate is as follows:
(1) When GI length>maximum delay, it is possible to avoid interference, but the transmission rate decreases;
(2) When GI length=maximum delay, it is possible to avoid interference, and the communication rate is good; and
(3) When GI length<maximum delay, deterioration due to interference occurs, but the transmission rate increases. Accordingly, it is desired to adjust the GI length so as to balance avoidance of interference and the transmission rate.
Patent Document 1 discloses an example of the communication system for adjusting the GI length. FIG. 13 is an example of a block diagram illustrating a configuration of an OFDM communication apparatus disclosed in Patent Document 1. In Patent Document 1, the guard interval length that is a key of efficiency of communication is configured to be variable and controllable. According to the abstract of Patent Document 1, following operations are disclosed.
A measurement symbol transmitting section 22 transmits a series of measurement symbols sequentially. A measurement symbol receiving section 23 sequentially receives the measurement symbols transmitted through a communication channel 21, and detects the shortest guard interval κ that can be identified. A notification transmitting section 25 notifies κ to a notification receiving section 26. The notification receiving section 26 outputs the notified κ to a data symbol transmitting section 30. Using the guard interval length κ, the data symbol transmitting section 30 generates a data symbol ω of OFDM from input transmission data X to transmit. Using the guard interval length κ, a data symbol receiving section 31 receives the transmitted OFDM data symbol ω, and outputs the reception data Y (citation from the abstract of Patent Document 1).    Patent Document 1: Japanese Laid-Open Patent Publication No. 2001-69110    Patent Document 2: Japanese Laid-Open Patent Publication No. 2003-143654    Non-patent Document 1: Kenkichi Hirade, Hiroshi Suzuki, Kazuhiko Fukawa, “RLS-MLSE for mobile radio communications”, IEICE, Technical Report, RCS98-126, p. 45-52, October, 1998